This proposal is to explore regulatory mechanisms in auditory perception using experiments in squirrel monkeys and cats. The experiments in squirrel monkeys will measure single unit events in a subcortical site of auditory pathway (the medial geniculate nucleus) while the animal is making judgments on a lateralization task using binaural time or intensity cues. The influence of selective attention to the various parameters of the acoustic signal and of ablation of portions of auditory cortex on both performance and unit activity will be examined as a means of relating afferent events in auditory pathway to auditory perception. Two regulatory systems affecting the encoding of responses in auditory pathway will be examined in acute studies in cats. The first will detail excitatory and inhibitory synaptic interactions in cochlear nucleus using intracellular techniques and define mechanisms of inhibition and the cell types responsible for the different synaptic events. The second approach will examine middle ear muscle regulation as it relates to motor systemr participating in the localization of a sound source. The experiments are part of a continuing effort to understand the interaction of efferent and afferent processes in auditory perception.